1. Field of the Invention
The invention relates to polyestercarbonate-siloxane copolymers and improved methods of their preparation.
2. Brief Description of the Prior Art
Polyestercarbonate-siloxane copolymers are well known polymer resins useful in a wide variety of applications; see for example the disclosures of U.S. Pat. Nos. 4,123,588 (Molari J.) and 4,569,970 (Paul et al).
The polyestercarbonate-siloxane copolymers are generally prepared on a commercial scale by copolymerization of monomeric precursors. In doing so, the carbonate precursors and the siloxane monomers are all charged to the reaction vessel and random copolymerization is effected; see for example U.S. Pat. No. 3,419,634 (Vaughn Jr.).
We have found that when an ordered copolymerization is carried out, in a step-wise sequence, the product copolymer resin is different in-kind, exhibiting enhanced thermal stability and contains a smaller concentration of low molecular weight species. The practical advantage of obtaining a resin with a smaller concentration of low molecular weight species is found in its processability. The resin, when thermally processed such as in molding operations exhibits a reduced "plate-out". The term "plate-out" as used herein refers to a phenomenon which often occurs during thermal processing of a polyestercarbonate-siloxane copolymer thermoplastic resin. The higher volatility of low molecular weight species associated with the copolymer causes them to "plate out" during thermal processing onto the molds and processing equipment, i.e., they condense on the surface of the molds, the processing equipment and also onto the surface of the molded article itself. The latter may affect quality of the molded article.